In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, an electrostatic latent image formed on a surface of a photoreceptor body by electrically charging the photoreceptor surface and exposing the photoreceptor surface to light is developed into a toner image, and a developing roller is used for the development.
More specifically, toner is electrically charged by rotating the developing roller in contact with an amount regulating blade (charging blade). The electrically charged toner is applied onto an outer peripheral surface of the developing roller, and the toner application amount is regulated by the amount regulating blade. Thus, a toner layer is formed on the outer peripheral surface of the developing roller as having a generally constant thickness.
When the developing roller is further rotated in this state to transport the toner layer to the vicinity of the surface of the photoreceptor body, the toner of the toner layer is selectively transferred from the toner layer to the surface of the photoreceptor body according to the electrostatic latent image formed on the surface of the photoreceptor body. Thus, the electrostatic latent image is developed into the toner image.
With a trend toward the use of a toner including more uniform, more spherical and smaller size toner particles or a polymeric toner, a semiconductive roller having a roller resistance controlled, for example, at not higher than 108Ω is effectively used as the developing roller in order to impart the toner with higher chargeability and efficiently develop the electrostatic latent image into the toner image without adhesion of the toner thereto.
Further, the developing roller is required to suppress degradation of the toner and have excellent imaging durability.
The term “imaging durability” is defined as an index that indicates how long the image formation quality can be properly maintained when the same toner is repeatedly used for the image formation.
A very small part of toner contained in a developing section of the image forming apparatus is used in each image forming cycle, and the remaining major part of the toner is repeatedly circulated in the developing section. Since the developing roller is provided in the developing section and repeatedly brought into contact with the toner, whether or not the developing roller can prevent damage to the toner is a key factor to the improvement of the imaging durability.
If the imaging durability is reduced, the formed image is liable to have white streaks in its black solid portion or have fogging in its marginal portion, thereby having a lower image quality.
To cope with this, it is contemplated to add a softening agent such as an oil or a plasticizer, or to use a liquid rubber in combination with other rubber as a rubber component (Patent Documents 1 to 3) to improve the flexibility of the developing roller.
However, the use of the softening agent or the liquid rubber is liable to increase the compression set of the developing roller. The developing roller having a greater compression set is liable to suffer from so-called permanent compressive deformation. That is, when the developing roller is kept in press contact with the photoreceptor body during the stop of the image forming apparatus and then is rotated to be brought out of the press contact, for example, the press contact portion of the developing roller is not restored to its original state. This may result in imaging failure such as uneven image.
Particularly, when the developing roller is incorporated in a developing unit of the image forming apparatus and kept in contact with the surface of the photoreceptor body for a longer period of time in a higher temperature and higher humidity environment, for example, the softening agent is liable to bleed on the developing roller. The bleeding softening agent is liable to contaminate the photoreceptor body to cause imaging failure (e.g., a contamination line occurs in a formed image).